The present invention relates to an apparatus for collecting water vapor, escaped fibrous matter, coat mist and dry fibrous dust occurring in the manufacture of paper and board or adhering the same to the web being processed.
The manufacture of paper and board can be divided into two phases comprising the formation of the base web, that is, the base board or paper, followed by the treatment of this web by coating, calendering, slitting and rolling. With the exception of rolling, all of these steps involve emission of different kinds of mist and dust that are detrimental if allowed to escape to the surroundings. Hence, a major portion of the paper machine and finishing equipment must be enclosed in hoods and enclosures. From the interior of such enclosed spaces, the mists are removed by suction with a vacuum. The air sucked off from the enclosed spaces is cleaned from moisture, dust and mist prior to discharging the air into the ambient atmosphere. Such enclosures as well as the cleaning of the sucked air are expensive to implement. The collection systems must be designed for large volumetric air flows, because the internal surfaces of the hoods and equipment must be kept free from condensation or dirt that could fall or otherwise land on the web being manufactured. Particularly in coating a paper web, water droplets or other foreign matter falling on the web can easily cause defects in the web being made. However, regions of insufficient flow velocity may remain in the hood structures that thus may allow accumulation of foreign matter in the system. Furthermore, the collection of coating mist in particular from the exhaust air is cumbersome and complex to arrange, because the collected waste coat cannot be dumped in the plant sewer system due to economical and environmental reasons, but rather, it is recirculated back to the machine circulation. However, as the coat returned to the machine circulation must be free from foreign matter and air bubbles, the collection of coat mist from an air flow is a technically challenging task.
It is an object of the present invention to provide a method suited for controlling the emissions of dusts, mists and other matter occurring in droplet or particle form in paper manufacture by virtue of guiding such matter to desired surfaces.
The goal of the invention is achieved by way of guiding the droplets or particles to be treated to impinge on a desired surface by means of both an electric field imposed between a counter-electrode taken to a low potential, advantageously to the ground potential, and a plurality of electrodes, advantageously having a pointed structures which are taken to an elevated potential, and additionally by the ion-blast wind induced by a corona discharge generated in the vicinity of the electrodes taken to the elevated potential. Such a collecting surface may be formed by, e.g., the web being made or, alternatively, a purpose-made ground-potential electrode.
The invention offers significant benefits.
By virtue of the invention, air-borne foreign matter can be collected directly to a desired surface which may be a web being treated or formed, for instance. In the case that the coat dust, or alternatively, the humid and fiber-containing mist emitted from the web being formed can be effectively returned to the web running in the process, the amount of foreign material to be removed by means of a vacuum will be reduced substantially and the cleaning of the exhaust air becomes easier. The mist emitted from the coating equipment can be collected directly to a counter-electrode (ground electrode), whereby the collection and removal of coating mist takes place in a single step. Since the mist adheres under electric forces to the collecting electrode, the coating mix layer thus formed contains less air than waste coating collected by conventional techniques from an air flow making the collected coat easier to return to the machine circulation. The design of the assembly is readily modifiable which is a great benefit as the installation space available in paper machines is extremely limited due to different reasons. By virtue of the assembly according to the invention, the emissions to be collected can be captured very close to their point of origin that helps to prevent the soiling of the paper-making equipment. A particularly advantageous benefit is the possibility of returning the collected material back to the web, whereby the amount of recirculating material is reduced.